Electrical: Question on 1947 Champion Radio

[avantibngrant]

I posted a while back on the wipers, and then the subject went over to the radio. I thought I should ask this question in the electrical section. I will start with a quote from RadioRoy from that post: If you replace the vibrator, you should also replace the buffer capacitor (the 1600 volt one) and the electrolytic capacitors in the power supply.

http://www.tubesandmore.com/products/P-V1015P

You can just throw a new vibrator in there if you like, but do not be surprised if it has a shorter lifespan than you anticipated.

Additionally, make sure the battery in your 47 is hooked up positive ground. Many solid state devices, like the new vibrators, die instantly and violently with reversed polarity.
I have the vibrator now and I also got a radio factsheet with the schematic. Looks like the buffer 1600 Volt one is the #13 one at 1600 Volts and .006 micro farads. The electrolytic one - is that #9 A,B,C, and D can capacitor with 4 Capacitors built in? If it is, would anyone (probably you Roy!) know of a good source for it? The Mallory # is FP424.
Thanks very much

Neil

[gordr]

Yes, the can capacitor is the multi-unit electrolytic. Some people open up the cans and re-stuff them with modern capacitors, which tend to be much smaller, so the hardest part of the job is forming a nice crimp on the can again. The 1600 volt buffer capacitor is usually soldered directly to the pins of the vibrator socket. It's generally good practice to replace all the paper capacitors in a radio set this old. They can start to leak DC current, which could really raise hell if it happens to be a coupling capacitor from the plate of one stage to the grid of the next. Capacitors are cheap; probably the full set for your radio will cost less than the vibrator. Here is a supplier near you: http://www.justradios.com/ I have bought from them several times.

What I do when I recap a radio is cut the leads of the old one to leave 1/4" stubs. I then trim the leads of the new caps (usually finer wire) and wrap the end of each lead around a wire former to form a tight coil of about 4 turns. Trim off the free end, and slip each of those coils over the old capacitor leads, and solder with 60/40 rosin-core solder. Avoid lead-free solder for radio work! This method takes less time, and avoids placing stress upon old tube sockets or terminal strips when trying to remove the leads of the old capacitors, those leads usually being tightly wrapped around the terminal before they were soldered.

[RadioRoy]

Since you have not redone a radio in some years, I would do this. Replace one capacitor at a time and fire up the radio in between replacements. that way, if you make a mistake, you will catch it immediately. If you replace all the caps and then fire up the radio, and made a mistake, you will be at a loss as to which one is bad, or not hooked up right.

I replace the filter caps with a common value, like 33 uf at 450volts and mount them on a solder type terminal strip. The one that bypasses the power amp cathode resistor only has to be about 25 volts max. It's OK to go over on the capacitance of these electrolytic caps. The ones in the rest of the circuit should be as close to existing design capacitance value as possible.

I like Gord's technique, too.

If you only have a battery charger as a power supply, hook it to a 6 volt battery, since the charger itself is unregulated and its voltage will vary widely.

[bezhawk]

Will tantallum (sp) capacitors work in place of the foil ones originally used?

[RadioRoy]

Yes. As long as the capacitance is the same and the voltage rating is equal or higher, they will be fine.

The only caution is to not use ceramic capacitors in a circuit that determines frequency, like the oscillator in the front end. Ceramic capacitors are inexpensive and work well, but their capacitance drifts with heat, That's fine for coupling and decoupling circuits, but anything that is part of a tank circuit should not have them.